1. Field of the Invention
This invention relates to a thin type inner rotor motor for driving rotation of a medium that can be used in a magnetic disk driving apparatus.
2. Description of the Related Art
Disk apparatuses are widely used in, for example, office computers and word processors including personal computers. One type of disk apparatus is shown in FIG. 14.
In FIG. 14, element 101 represents a chassis having a spindle center 102 as a disk rotation center, which is installed in a housing (not shown) of, for example, a personal computer. A slot is formed by a box with a bottom which is opened forward and upward and has a housing space to which a disk cartridge 103 approaches.
At a rear end of the chassis 101 are disposed a stepping motor 124 for forwarding a head carriage and a head carriage which is configured to be movable in a cross direction by the stepping motor 124. At the tip of the head carriage is held a first head 130 which reads recorded information on a disk. At a rear upper end part thereof, a head arm 132 having a second head 131 which corresponds to the first head 130 through an elastic body is mounted such that it can oscillate. This head arm 132 is biased in such a direction that the second head 131 approaches the first head 130. In the disk apparatus of this example are disposed a cartridge holder 136 which holds the disk cartridge 103 removably and a mechanism which opens and closes a shatter of the disk cartridge 103.
To reduce the thickness of this type of disk apparatus, an inner rotor motor as shown in FIG. 15 has been used as a motor for rotating a disk. The disk comprises a stator 164 having a circular yoke 161 extending in the circumferential direction and a plurality of cores 163 which are disposed on an inner peripheral surface of the yoke 161 in a radial pattern and on which coils 162 are wound. A rotor 166 is rotatably disposed in an inner peripheral part of the stator 164. A circular magnet 165 lies opposite the cores 163. In the figure, a holding part 170 that houses a bearing 169 is mounted on a circuit board 168. A rotation shaft 171 fixes the rotor which is rotatably supported by the holding part 170 on the circuit board 168 through the bearing 169 and has an axis line extending upward and downward. In addition, the rotor 166 of this inner rotor motor functions as a turntable which has a magnet (not shown) and a turning lever (not shown) for chucking a disk.
In this kind of stator for use in the inner rotor motor, the yoke 161 and cores 163 are disposed to surround nearly the entire circumference of the round shaped rotor 166 except for a movement zone of the heads 130 and 131, and are made for example, of silicon steel which is expensive compared to galvanized sheet iron which forms the chassis 101.
Presently, there exists a demand to reduce manufacturing cost and a strong demand to reduce the size and weight of disk apparatuses. Therefore, there is a demand to reduce the areas of the yoke 161 and cores 163, which are made of expensive silicon steel, in the stator used in the inner rotor motor.
However, if the yoke 161 and the cores 163 are reduced, the magnetic interaction with the rotor 166 becomes uneven along a circumferential direction so cogging torque was often generated. Cogging torque triggers defects such as a reduction in the torque of the rotor 166, rotational irregularity of the rotor 166, and increase of control currents for compensating the rotational irregularity. Thus, the cogging torque has to be reduced as much as possible.
Cogging torque will be described with reference to a schematic diagram of a motor shown in FIG. 16. In FIG. 16, 501 represents a circular magnet rotor magnetized with multipoles and 502 represents a stator core having three magnetic teeth 502a to 502c. Coils 502d are wound on the respective magnetic teeth 502a to 502c. 
In FIG. 16, magnetic fluxes pass from an N pole near the magnetic teeth 502a toward an S pole near the magnetic teeth 502b. Magnetic fluxes pass from an N pole near the magnetic teeth 502c toward an S pole near the magnetic teeth 502a and 502b, respectively. When the total number of magnetic fluxes of N poles and S poles in the stator core 502 are made equal, cogging torque is reduced.
However, in an actual motor, due to, for example, magnetization irregularity of a magnet, dimensional accuracy of respective components and influence of a magnetic body disposed around the magnet, the total number of magnetic fluxes of N poles and S poles are not equal so cogging torque was often generated.
The present invention provides a motor that can reduce cogging torque and maintain rotation stability.
The invention employs the following structure.
A motor of the invention comprises a rotor having a plurality of magnetic poles disposed in an arch or circular shape, and a stator in which coils are disposed on respective magnetic teeth of a stator core having a plurality of the magnetic teeth disposed outside or inside of the circumference and opposite the rotor, wherein a magnetic pole part for cancelling cogging torque is disposed around the rotor.
Since the magnetic pole part for cancelling cogging torque is disposed around the rotor, the cogging torque of the motor can be canceled because of the magnetic interaction between the magnetic pole part and the rotor.
Further, the stator is disposed at an outer peripheral side of the rotor covering a range of within about 180xc2x0 relative to a center angle of the rotor. The magnetic pole part is disposed at an opposite side to the stator by sandwiching a center of the rotor.
According to such a motor, since the magnetic pole part is disposed at the opposite side to the stator by sandwiching the center of the rotor, the magnetic pole part doe not interfere with the magnetic interaction between the rotor and the stator. Thus, it becomes possible to configure a motor which reduces rotation irregularity.
Furthermore, the rotor is supported rotatably on a surface of a base made of a ferromagnetic material through a rotation axis. A tip of the magnetic teeth of the stator is disposed at a position from which is viewed a cutting portion disposed on the surface of the base and which is opposite the outer peripheral surface of the rotor. A tip of the magnetic pole part is disposed at a position from which is viewed another cutting portion disposed on the surface of the base and which is opposite the outer peripheral surface of the rotor.
According to such a motor, since the tip of the magnetic teeth and the tip of the magnetic pole part are disposed at a position from which is viewed the cutting portion of the base, at a portion where the magnetic teeth and the magnetic pole part are located, magnetic fluxes from the rotor affect only the magnetic teeth and the magnetic pole part and do not affect the base. Therefore, the generation of cogging torque due to the operation of the base and the rotor can be reduced.
Moreover, in the motor of the invention, the magnetic pole part is plate-shaped. An end face which is located at the tip of the magnetic pole part is made to be a curved surface along the outer peripheral surface of the rotor.
According to such a motor, since the end face of the platy magnetic pole part is made to be a curved surface along the outer peripheral surface of the rotor, magnetic fluxes from the rotor can be effectively applied to the magnetic pole part so cogging torque can be reduced.
Further, in the motor of the invention, a center position in a thickness direction of the tip of the magnetic teeth and a center position in a thickness direction of the end face of the magnetic pole are disposed at an identical position along the rotor""s rotation axis.
According to such a motor, since the magnetic teeth and the magnetic pole part are disposed at an identical position along the rotor""s rotation axis, the rotor is not inclined to the rotation axis so the rotor can stably rotate.
The disk apparatus of the invention can be used with any of the motors described above for use in driving the rotation of a disk.